1. Field of the Invention
The present invention relates to hydraulic braking systems, particularly such systems operable in conjunction with a regenerative braking system employed in an electric powered or a hybrid vehicle.
2. Description of the Related Art
Dynamic braking to quickly stop an electric motor and any load it may be powering is an old and well-known technique which converts the mode of operation from that of a motor to that of a generator. Traditionally, the kinetic energy of the system is dissipated in a braking resistor as heat. The effectiveness of dynamic braking is dependent on the ohmic value of the braking resistor and on motor speed, falling off at lower speeds.
Rising energy costs and environmental concerns have lead to the recent popularity of electric and more especially hybrid vehicles which are powered at least part of the time by an electric motor operating from an onboard storage battery. These vehicles provide the opportunity to operate the electric motor as a generator to slow the vehicle. With conventional friction braking or traditional dynamic braking, the kinetic energy of the vehicle is wastefully dissipated a heat. Utilizing the electric energy generated by the motor during dynamic braking to recharge the onboard battery (known as regenerative braking) is far more energy efficient. Unfortunately, concerns such as the speed-dependent efficiency prohibit the exclusive use of regenerative vehicle braking. Typically, the vehicle is also equipped with hydraulic brakes.
As the effectiveness of regenerative braking diminishes with vehicle speed, more reliance must be put on the hydraulic system. Also, it is desirable to blend the effects of regenerative braking and hydraulic braking in a manner undetectable by the driver. U.S. Pat. No. 4,425,005 teaches a vehicle braking system having one brake circuit actuated directly by pressure generated in a master cylinder and a second brake circuit actuated by pressure generated by actuation of the master cylinder to the extent that regenerative braking effort acting on the wheels having brakes in the second brake circuit is insufficient to provide the desired total braking effort on those wheels. When regenerative braking on those wheels is sufficient to meet the braking effort demand, the hydraulic brake system does not supply any additional braking effort to those wheels. A compliance control member provides the feel of input travel and force equivalent to the regenerative braking effort actually at the wheels of the second brake circuit when regenerative braking is occurring during vehicle braking. The absorbed displacement and force is fed back to the second brake circuit by the compliance control member when the regenerative braking effort becomes insufficient to meet the braking effort demanded from the brakes of the second brake circuit.
A more recent effort is disclosed in U.S. Pat. No. 6,176,556. Here, a vacuum boosted hydraulic brake system diminishes the amount of vacuum boost in proportion to the amount of regenerative braking. One drawback of this system is that the hydraulic system is operable on all four wheels while the regenerative system is employed on only two of the wheels. The system diminishes the overall vacuum boost which could lead to disproportionate braking between the front and rear.
Current boosters for regenerative braking systems typically include a pedal simulator, a boosted power piston, and a gap between them to allow for manual push-through in case of a loss of boost pressure. During regenerative braking, the pressure to the brakes must be reduced as traction braking is increased. At the same time, brake pedal travel and force should be consistent with the driver's deceleration request and independent of the mix of hydraulic and regenerative braking. The pedal simulator provides this decoupled pedal feel. In most systems, the boost pressure generated is proportional to the pedal input force. Valves in the downstream ESP (Electronic Stability Program) hydraulic unit are used to hold off, reduce, and increase the brake pressure at various times during a stop, providing the blending function.